The present invention relates to cryostats, and in particular, to cryostat sample holder arrangements.
A cryostat is a vessel which allows a sample to be maintained at a very low temperature, for example, temperatures less than 0° C. The sample may then be subjected to experimentation in the cryostat at the very low temperature. The sample is placed on a sample holder or sample plate which is usually cooled to low temperatures by boiling a low temperature liquid into a gas, for example, by boiling liquid nitrogen at −196° C., or liquid helium at −269° C. In addition to cooling the sample, it is also often possible to manoeuvre the position of the sample in the cryostat by rotating the sample plate. This is desirable in spectroscopy experiments, as it allows the probing beam to remain static and the sample rotated, so as to alter the incident angle of the probing beam on the sample.
FIG. 1 shows a cross-sectional view of a known cryostat. A sample that is to be cooled is placed on to a sample plate in the cryostat, and is then subjected to cooling by boiling a low temperature liquid. The liquid is boiled in a gas tank, which is thermally coupled to the sample plate, by a thermal conductor. The conductor extends from the tank, around the outside of the sample plate movement apparatus, to the sample plate.
In the prior art cryostat shown, the sample plate and hence, sample, is rotated by means of a belt which extends around a pulley system, with a main drive shaft being mounted underneath the centre of the sample plate, i.e. directly on the rotational axis of the sample plate. However, such a drive arrangement has a number of disadvantages.
The use of a drive belt induces a slack into the system, such that it can be difficult to accurately control the rotational angle of the sample plate. Furthermore, the arrangement of the drive belt and the pulley system is relatively bulky, which means that a radiation shroud that is placed over the sample holder and sample has to be quite large in order to allow the equipment to be enclosed therein. In addition, the bulky equipment increases the amount of heat loss from the cryostat apparatus, such that it is not always possible to cool the sample to the very low temperatures that are often required in experiments.
In an attempt to address some of the above-mentioned problems inherent with using a pulley system to drive a cryostat sample holder, some manufacturers have replaced the pulley system with a bevel gear system, for example, a ‘straight bevel’, ‘spiral bevel’, or ‘zerol bevel’ gear system. Each of these types of bevel gear system consists of a rotating ‘drive gear’ or ‘pinion’, which is arranged to drive a ‘driven gear’ attached to one side of the sample holder. The axis of rotation of each gear is arranged such that it is along the radius of the other gear in the system. The axis of such bevel drive gears extends along the “zero line” of the driven gear, i.e. radial with respect to the driven gear. Hence, such bevel gear systems are referred to as “intersecting axis gears”.
However, bevel gears tend to suffer from backlash. Backlash is the amount of “free play” in the gearing system. Bevel gears can also generally be driven backwards. This means that occasionally the drive gear and driven gear drive backwards due to torque acting on the driven gear. Hence, torque on the driven gear causes the driven gear to turn the drive gear itself, at least partially, and this decreases the accuracy with which the sample holder (and hence sample) can be rotationally positioned.
Other manufacturers have used rack and pinion gears, which suffer from similar problems.